Telephone system



June 20, 1933. c. R. BROOKS 1,914,358

TELEPHONE SYSTEM Filed Nov. '7, 1931 2 Sheets-Sheet 1 Imm? Cher-les E. Braak' maf-M June 20, 1933. C, R BROOKS 1,914,358

TELEPHONE SYSTEM Chaz-1E: E. :E1-'asks Mvm Patented laine ltll stares earlier orales CHABLTS BRGGKS, GF CHICAGO, LLNOS, JISSGNOR, BY JIESNE ASSGNMENTS, TO

ASSOCATFB TN OF DELAW'ARE ING., F C-ICAGO, LLENOIS, A CORPORA- TELEPI-IONE SYSTEM Application le. November '7, 1.931. Serial No. 573,662.

ne present invention relates to telephone systems in general, but is concerned more particularly with automatic telephone systems used to give service to subscribers in towns and villages and in the surrounding' rural districts.

The main object of the present invention is the .production of new and improved circuit arrangements for enabling satisfactory telephone service to be given to the rural lines entering the town or village exchange., even though many of the rural lines have a high resistance and be in a relatively poor state of repair.

This application is an improvement of my prior application, Serial No. 490,877, filed oel-.Ober 2l, i930.

General description As pointed out in my prior application above referred to, the control of automatic switches from substations on rural lines is quite a difficult `problem and one which rcquires special apparatus in its solution. ln

' the system disclosed in my prior application, each single-wire rural line is provided with an individual repeater which responds to impulses received from the rural line and repeats standard loop impulses to the automatic switches, having for this purpose vacuum-tube apparatus responsive to applications of potential from the transmitter' battcry at the rural substation, as controlled by the usual calling device.

ln this prior application, the potential impressed on the line from the transmitter battery at the substation is employed also to control the release of the established connection, as well as to give answering supervision when a call received on the rural line is answered. Y

rlhe present invention has to do with improvements on the prior system, briefly outlined above, and it contains the following outstandingfeatures l. In order to provide a more reliable arrangement to control the transmission of the dial impulses to the automatic switches, the calling device on the rural line is arranged to control the generation of impulses of alternating current, which are transmitted over t-he line to the repeater and are responded to by a tuned device which controls the vrepeating of standard loop impulses to the automatic switches.

In connection with this feature, arrangements are provided for shunting the contacts of the tuned relay in the repeater duri'g conversation in order to prevent disturbing clicks and possible release ofthe established connection in case the tuned relay responds to voice currents.

2. ln order to provide a more reliable holding and releasing arrangement, and one which does not depend entirely upon the momentary impulses or surges produced by removing and replacing the receiver', advantage has been taken of the double-frequency alternating-current ripple transmitted from the first tube of the repeater to the second tube by arranging the circuit arrangementassociated With the tubes so that the value of the plate current flow at the second tube depends upon the strength of the alternating ripple current transmitted to theA second tube from the first, the volume of which ripple is dependent upon whether the receiver is on or off at the substation.

3. ln addition to the foregoing' features, the arrangement for draining the rural lines after an application of ringing current thereto has been improved and made more certain by the addition of a slow-releasing line-drain relay, and other operations of the repeater have been simplified and rendered more reliable, as Will hereinafter appear.

Referring now tothe accompanying drawings, comprising Figs. l and 2, they show by means of the usual circuit diagrams a sulicient amount of apparatus in a telephone system involving the features of the invention to enable the same to be understood.

Fig. l shows a rural single-eonductor party line l and one of the substations, Al, thereon; together with the repeater R in the exchange, individual to the party line 1.

Fig. 2, which should be placed to the right of Fig. l, with the corresponding conductors at the edges of the sheets in alignment, shows automa-tie switching apparatus which may be used in setting up connections to and from the party line 1 of Fig. 1.

The line switch LS of Fig. 2 is a regular two-wire rotary line switch, of which only the Wipers 203-205 are shown.

The selector S, of which only the wipers are shown, may be any standard two-wire selector of the type having the bank contacts arranged in horizontal rows or levels.

The connector C1 is of the same mechanical type as the selector S, and it may be the same as the connector C1 in Fig. 3 ot the Saunders Patent No. 1,686,879, granted October 9, 1928, except that the relay 301 of the connector C1 in the present drawings is arranged to operate over a loop circuit when the subscriber responds; and the ringing current is supplied to the called line by the present connector over the lower line wiper instead of over the upper line wiper as in the Saunders patent. The connector C1 has access to a. number of common battery lines, such as the line of substation A2, and it has access to a number of local battery rural lines, such as line 10 of substation A1, Fig. 1.

Detailed description The invention having been described generally, a detailed description ot the operation of the apparatus shown will now be given. For this purpose, a call from substation Al of Fig. 1 to substation A2 of Fig. 2 will now be explained.

@all from substation-11 to substation- 15J lVhen the subscriber at substation A1 desires to call the subscriber at substation A2, he must first project ringing current over his line to the exchange to condition the re. peater R to respond to the removal of the receiver.

Preparing the repeater R for operation lVhen the subscriber at substation A1 manipulates his hand generator, ringing current flows over the line 10 to the repeater R, operating the alternating-current relay 6. Relay 6 is enabled to remain operated continuously 'from the flow of alternating current by virtue of the rectifier 14 which by-passes the relay 6 during one-half oi' each cycle, preventing current reversals in the relay.

Upon operating, relay 6 closes a circuit for the timing relay 7, whereupon relay 7 operates and attracts its flexible, weightedpendulum armature, thereby closing a circuit for timer relay 8. Then relay 8 operates, it disconnects conductor 1 of the calling line from the grid element of tube 21 at its lower armature, and at its inner-lower armature, and at its upper armature it cieses circuits for both windings ot' the slow-acting relay 9, at the same time placing a guarding potential on conductor 'l, Fig. 2, by way of conductor 51 of Fig. lathereby guarding the calling line against seizure by the connector C1.

When relay 9 responds to the fiow of current through its windings, it prepares a cir- Acuit for impulse relay 11 at its inner-lower armature; connects up the plate battery 28 at armature 15; closes a filament-lighting circuit for the tubes 21 and 22 at armature 17 and at armature 16 energizes the relay 29 to connect the commercial power to the primary Winding of the filament transformer 30. The filaments of tubes 21 and 22 new become lighted, and plate current from battery 28 now flows through resistance element 23 in the plate circuit of tube 21 and through the marginal holding relay 12 in the plate circuit et' tube 22. Because of its marginal adjustment, relay 12 is unable to operate because the plate current in tube 22 is relatively low at this time, for reasons which will be pointed out hereinafter.

Upon the cessation of ringing current, relay 6 restores, opening the circuit of the timer relay 7 and closing an additional point in the circuit of impulse relay 11. When the timer relay 7 deenergizes, the flexible weighted armature thereof is freed and starts to swing rapidly back and forth with a pendulum-like motion of a slowly decreasing amplitude. As long as the armature of relay 7 is vibrating with suflicient amplitude to close the circuit of relay 8 at the limit of each swing, relay 8 remains operated because of its slow-releasing characteristic.

During the time required for the oscillatory motion of the armature of relay 7 to die down and release relay 8, the calling subscriber is expected to remove his receiver and impress the negative potential of the transmitter battery at substation A1 on line conductor 3. This potential does not have any effect on the repeater R for the time being, however, because conductor 1 is maintained disconnected from the grid element of tube 21 at the lower armature ol' relay 8.

CZosz'ng t/Le switch-control @treu/t transmitter battery being connected in shunt ofthe grid leak resistance 24. The placing of the additional negative potential on the grid element of tube 21 results in an increase of the positive potential of the plate element of the tube because of the increased effective impedance between the plate and filament elements of the tube. This transmits a momentary positive impulse to the grid element of'l tube 22 by way of condenser 26, causing a momentary increase in the current in the plate circuit of tube 22 for the duration of the interval required for the normal potential of the grid element of tube 22 to be restored by current flow through the grid leali resistance element 27.

The alternating ripple cur/rent In addition to the momentary increase in current flow in the plate circuit of the tube 22, as explained above, the increased negative potential on the grid of the tube 21 brings about an increase in the flow of plate current in the tube 22 which endures as long as the added negative potential exists on the grid element of the tube 21. This latter effect is produced because the lowered plate current in the tube 21 results in a substantially smaller amount of alternating ripple current being transmitted by way of condenser 26 to the grid of the tube 22. The value of the grid biasing battery 31 and the value of the grid-leak resistance element 27 are so chosen, with reference to the characteristics of the tube 22, that the tube 22 acts in somewhat the same way as the detector tube in a radio receiver which increases the How of plate current with a decrease in the flow of the incoming carrier wave, as is well known.

Regarding the generation of the abovementioned alternating ripple current, it is apparent that the alternate heating and cQoling of the filament of the tube 21, which occurs twice during each cycle of the filament lighting current, causes a periodic increase and decrease in the iilament emission of tx 1ice` the frequency of the filament-lighting current. In addition, the filament potential is continuously varied with respect to the fixed grid potential. Nom-since the mid-point of the secondary winding of the transformer 30 is grounded, the mid-point of the filament of the tube 2l remains at ground potential, causing one-half of the filament to be positive with respect to ground potential, while the other half of the filament is negative with respect to ground potential. By operating the tube 21 with the grid potential near one end of the straight portion of the grid-response curve, the decrease in emission from one-half the filament at maximum lament potential is not balanced by trie increase in emission from the other half the filament, giving an additional alternatingcurrent ripple in the plate current, which may assist or oppose the one first described, depending upon the potential at which the grid is normally maintained with respect to the average potential of the filament.

Now, when the filaments of the tubes 21 and 22 are first lighted and the source of plate current 28 is first connected up at armature 15, the alternating current transmitted through the condenser 26 from the tube 21 of the grid element of tho tube 22 is at a maximum, causing the plate current to the tube 22 to be at a minimum, by reason of the hereinbeforementioned well-known detecting or rectifying action. Then, when the calling line 3-on which the negative potential of the transmitter battery at substation A1 has been impressedis connected up at the lower armature of timer relay 8 to the grid circuit of tube 21, the more negative potential impressed on the grid element lowers the plate current flowing through the tube 21 to such an extent that the alternating current transmitted to the grid element of the tube 22 is materially reduced, permitting the average value of the plate current through tube 22 and the marginal relay 12 to be materially increased. Vhen this increased current flow takes place, accompanied by the momentary increase hereinbeforementioned, relay 12 operates and remains operated as long as the additional negative potential remains impressed upon the grid element of the tube 21. It is to be noted that relay 12 is provided with a copper collar at the heel end thereof, as indicated by the shaded lower portion of the relay 4. This copper collar renders the relay slightly slow in releasing, enabling it to remain operated during momentary disturbances in the flow of plate current.

Operating the Zineswz'tc/i When relay 12 operates as above mentioned, it disconnects ringing relay 3 from conductor 50 at its upper armature, at the same time preparing a bridge across conductors 50 and 52, and at its lower armature it completes a circuit for impulse relay L1 by way of' the contacts of the 100G-cycle, tuned relay 13, and contacts of relays 9 and G. lVhen this circuit is closed, relay 11 operates and connects the calling line to the lefthand repeating-coil windings at its upper armature, and at its inner armature it closes the bridge across conductors 50 and 52 by way of contacts of relay 12 and the upper winding of the electro-polarized relay 10. Owing to the direction in which electropolarized relay 10 is polarized by the lower winding, current flowing through the upper winding in the normal direction does not operate the relay.

As a result of the closure of the bridge across conductors 50 and 52, the line switch LS responds in the usual manner to select an idle trunk line, which trunk line, it may be assumed, is the trunk line ext-ending to the selector S. When this trunk line has been selected, the conductors 50-52 are extended by way of wipers 203-205 to the corresponding conductors extending to the selec-- tor S. As soon as the usual line relay of the line switch has operated, ground potential is applied to conductor 5l, thereby closing a holding circuit for slow-releasing relay 9.

It is common practice to provide a source of dial-tone current at the selector, from which dial-tone current is transmitted back to the calling line when the selector is seized, thereby informing the calling subscriber that he may proceed to dial the digits of the desired number.

D'z'alh'ng the desired number lVhen the calling subscriber manipulates his calling device in accordance with the first digit of the desired number, the olinormal contacts of the calling device CD shunt the receiver and transmitter as soon as the dial is moved from its normal position. As the dial of the calling device is returned to its normal position, the impulse contacts thereof separate a number of times, depending vupon the digit being dialled. Each time these contacts separate, the shunt is removed from around the 1000- cycle, tuned buzzer 2, causing a 1000-cycle interruption in the flow of transmitter current to take place. This 1000-cycle current is repeated at the induction coil., causing a 1000-cycle current to flow between ground at the calling substation and the grid element of the tube 21. This 100G-cycle current is amplified and repeated through the tubes 21 and 22 to the 1000-cycle, tuned relay 13, by way of the associated condenser. The tuned actuating spring of the relay 13 is caused to vibrate with suiiicient amplitude to open the circuit of the impulsing relay 11 on each outward swing of the vibrating armature spring of relay 13. Relay 11 is adjusted with a fairly stiff spring tension so that it restores the first time its circuit is opened and does not again operate as long as its circuit is being interrupted at the contact of relay 13.

lVhen the impulse springs at the calling device CD close at the end of the first impulse, the tuned buzzer 2 is shunted and ceases its operation. When this occurs, the tuned relay 13 ceases its operation, permitting impulse relay 11 to operate again.

The foregoing operation takes place each time the impulse springs of the calling device CD separate.

Then the second, third, and fourth digits in the desired number are dialled,a second, a third, and a fourth series of impulses of 1000-cycle current are transmitted over the calling line to relay 13, causing additional series of impulses to be produced in conductors 50 and 51 by the impulse relay 11.

The selector S responds to the first series of impulses by raising its wipers opposite the desired level of bank contacts in the usual manner, following which it selects an idle trunk line by rotating its wipers across the selected level of bank contacts. Assuming that the connector C1 is the lirst found to be idle, this connector is taken into use and is prepared for operation in the usual manner by the energization of relays 302 and 303.

Responsive to the second and third series of impulses, the connector C1 responds by raising its wipers opposite the desired level of bank contacts and in rotating them over the contacts until they come to rest on the contact set associated with the line of substation A2.

Assuming that the line. of substation A2 is idle when called, connection is made to the line in the usual manner and the code selector CS is prepared for operation in response to the fourth series of impulses, transmitted from the repeater R in the same manner as the preceding series. As soon as the code selector CS has been positioned, ringing relay 307 of the connector C1 actuated in accordance with the selected code to apply ringing current to the line of substation A2 from the generator G2.

The called subscriber responds When the subscriber at substation A2 responds to the ringing of his bell by removing his receiver, back-bridge relay 301 operates to terminate the ringing operation. Relay 301 also reverses the connections bctween the incoming trunk conductors and line relay 302, thereby reversing the current flow in the talking conductors between the connector C1 and the repeater R.

In the repeater R, the electro-polarized relay 10 now operates and closes a local holding circuit for the impulse relay 11, thereby preventing this relay from responding to any further operations of the tuned relay 13, which sometimes takes place responsive to amplified voice currents.

The two subscribers may now converse With each other as desired, the talking circuits being outlined by the heavy conductors.

The release 0f the established connection lVhen the subscriber at substation A2 rcplaees his receiver following the conversation, back-bridge relay 302 falls back, reversing the current flow in the talking conductors to normal direction, whereupon electro-polarized relay 10 in the repeater R restores, but its restoration has no particular eifect at this time.

When the receiver is replaced at the calling substation A1, the usual transmitter circuit and receiver circuit are opened, whereupon the grid potential at tube 21 is rendered less negative. This permits an increased flow of alternating ripple current to take place in the plate circuit of tube 21, causing an increased flow ot' alternating current to the grid of tube 22. As explained before, this decreases the average flow of plate current through the plate circuit ot tube 22 suiiciently to bring about deenergiZation ot the marginal adjusted relay 12. When relay 12 :falls back, it opens the circuit of relay 11, and relay 11 restores. lvVith relays 11 and 12 restored, no bridge exists across conductors 50 and 52, allowing line relay 302 of the connector C1 to fall back and permit release relay 303 to restore. The connector C1 is now restored to normal position in the usual manner, and the selector S and the line switch LS are released responsive to the removal of ground potential from the release trunk conductor at the contacts of release relay 303.

In the repeater R, start relay 9 falls back shortly after ground potential is removed from conductor 51 at the contacts ot relay 303 of the connector. Upon 'falling back, start relay 9 extinguishes the iilaments of the tubes 2l and 22 and disconnects the plate battery 2S from the plate circuits ot the tubes.

The entire connection is now released and the apparatus involved therein is in condition for use in establishing subsequent connections.

@all from substation A2 250 substation A1 Assuming now that the subscriber at substation A2 desires to converse with the subscriber at substation A1, the line switch (not shown) associated with the substation A2 responds to the removal of the receiver by selecting an idle selector in the usual e5 manner, which selector may be assumed to be the selector S.

It may be assumed that the selector S takes the connector C1 for use when the irst digit is dialled, in which case the 50 connector 1 responds to the second and third digits in the desired number by bring ing its wipers into connection with the bank contact terminating conductors 50', 51', and

52; branches, respectively, of conductors 55 50, 51, and 52. When this connection 1s made, and ground potential is applied to conductor 51, a circuit is closed over conductors 51 and 51 for the upper winding of start relay 9. Start relay 9 thereupon 60 operates and energizes the tubes 21 and 22 as hereinbefore explained.

Responsive to the dialling of the fourth and last digit of the number, the code selector CS is positioned in accordance with 65 the code assigned to substation A1, 'following which ringing relay 307 is operated in accordance with the assigned code to transmit ringing current to the called line. The applications of ringing current made at the lower contacts of relay 307 to conductors 52 and 52 are oi no eilect at this time, as conductor 52 is opened at the repeater R. The accompanying applications or' ground potential to conductors and 50 results in a corresponding operation of ringing relay 3 to transmit ringing current from the generator G1 to the party line 1 in accordance with the operations of ringing relay 307.

It is to be noted that ringing relay 3 disconnects line 1 trom the repeater R each time it operates so as to avoid transmitting ringing current to the alternating-current relay 6 and to the grid element of tube 21. As a further result of each operation of ringing relay 3, the line drain relay 4 is actuated to disconnect conductor 1 from the repeater R at another point and to place a ground potential on the resting contact ot the upper armature of relay 3. Relay 4C remains actuated for a slight interval 'following each restoration of relay 3, whereby a momentary ground potential is placed on conductor 3 so as to drain or discharge the line and ringer condensers on the line before re-connecting the line to the repeater.

The first time relay l operates, it closes a circuit for the holding relay 5, whereupon relay 5 operates and closes a seit-locking circuit for itself at its upper armature, through the upper armature oi start relay 9. At its lower armature, relay 5 closes a circuit for impulse relay 11, thereby operating this relay and maintaining it operated continuously until the connection to the repeater R and its associated line is broken down. This operation is brought about to prevent relay i1 from responding lto voice currents during conversation, as explained in connection with the holding circuit tor relay 11 closed by the electro-polarized relay 10.

vWhen the subscriber at substation A1 responds to the ringing of his bell by removing his receiver, the placing of the additional negative potential on line 3 and the connected grid element oi' tube 21 results in the hereinbefore-described increased flow ot plate current at the tube 22. When this occurs, the marginally adjusted relay 12 operates and disconnects the ringing relay 3, at the same time closing the bridge across conductors 50 and 52. lVhen this occurs, back-bridge relay 301 operates, and the application of ringing current at the connector C1 is terminated in the usual manner.

Release of the established connection When the subscriber at substation A1 replaces his receiver following the conversation, the potential at the grid element of tube 2l is rendered less negative, whereupon the hereinbefore described reduction in the flow of plate current at tube 22 takes place and relay l2 restores. The restoration of the relay l2 results in the opening of the bridge across conductors 50 and 52 and the consequent deenergization of back-bridge relay 302 of the connector C1.

Vhen the receiver is replaced at the calling substation A2, the established connection is released and ground potential is removed from conductor 5l. When this occuis, start relay 9 of the repeater R falls back and opens the circuit of the holding relay 5, whereupon relays 5 and l1 restore. The established connection is now released and the apparatus involved therein is completely restored to normal.

What is claimed is:

1. In an automatic telephone system, automatic switching apparatus arranged to set up connections from a calling subscribers line to called lines, an alternatingcurrent generating device on said calling subscriber-s line, means tor controlling said alternating-current generating device to cause trains of impulses of alternating current to be transmitted from said device over said line, each train of impulses corresponding in the number of impulses it contains to a separate digit-in the number of the desired called line, and means responsive to said trains of impulses of alternating current for operating said automatic switching apparatus to set up a connection from said calling to said desired called line.

2. In an automatic telephone exchange system, a telephone line entering an exchange and having a normally inactive alternating-current generator thereon, a calling device on said line arranged to be operated in accordance with digits in a desired telephone number to render said generator active a desired number of times to send a like number of impulses of alternating current over the line to the exchange, and means in the exchange responsive to said impulses of alternating current for setting up a connection from the calling line to the desired called line.

3. In a telephone system, automatic switching apparatus arranged to extend a connection from a calling telephone line, means responsive to the application o'' a direct-current potential to said line for closing a switch-control circuit for said automatic switching apparatus, means on said line for transmitting impulses of alternating current over the line, and means in the exchange responsive to said impulses of alternating current for operating said automatic switching apparatus.

4. In a telephone system, a telephone line having a local-battery substation thereon and extending to a telephone exchange, an alternating-current generating device at said substation powered by the local battery, a calling device at the substation arranged to cause said alternating current generating device to transmit series of directive impulses over said line to the exchange, and automatic switching apparatus in the exchange responsive to said impulses to extend a connection from said line.

5. In a local-battery subscribers substation for use in a telephone system, a local battery for energizing the transmitter, a self-interrupting device arranged to be operated from said local battery, and circuit connections for causing said self-interrupting device to be operated intermittently to cause intermittent impulses of alternating current to be sent out from said substation.

6. In combination, a local-battery substation for use in a telephone system, hand-operated ringing-ciurent generator, a local battery for energizing the transmitter, a self-interrupting device arranged to be operated from said local battery, circuit connections for causing said self-interrupting device to be operated intermittently to cause intermittent impulses of alternating current to be sent out from said substation, an automatic telephone exchange, a line connecting said substation to the exchange, automatic switching apparatus in the exchange arranged to extend said line, means responsive to current flow from said ringing current generator for preparing said automatic switching apparatus for operation, and means responsive to said impulses ot alternating current for operating said automatic switching apparatus to extend the line.

7. In an automatic telephone system, a subscriber-s line entering yan automatic exchange, means at the exchange responsive to an application of ringing current to said subscriber-s line from a generator on the line to prepare a switch-control circuit, means responsive to an application of direct current made over said subscribers line from a source of direct current on the line for closing said switch-control circuit, and means responsive to series oi aiLiplications o'l alternating current from a separate current source on the subscribers line for intermittently interrupting the switch-control circuit.

8. In an impulse repeater arranged to control the operation of automatic switching apparatus by means of a control circuit. a di rect-current relay and an altermiting-current relay connected in a common circuit, means for transmitting direct current through said circuit to operate the directcurrent relay to close said switch-control circuit, and means responsive to impulses olf alternating current being superimposed on said direct current for operating said alterllo' Lle() i, l,ting current relay to produce eective interruptions in said switch-control circuit.

9. In an automatic telphone system, automatic switching apparatus to be operated over a control circuit to set up a connection trom a calling line to a called line, a repeater interposed between the calling line and the automatic switching apparatus, said repeater being responsive to impulses of alternating current controlled over the calling line for delivering switch-control impulses to the automatic switching apparatus over said control circuit, and means ett'ective atter the automatic switching apparatus has been positioned on a called line for rendering said repeater non-responsive to transmit further impulses over said switch-control circuit responsi1 e to further impulses of alcrnating current.

l0. 1n an automatic telephone system, automatic switching apparatus arranged to be operated over a cont-rol circuit to set up a connection from a calling line to a called line, a repeat-er interposed between the calling line and the automatic switching apparatus, said repeater being responsive to impulses of alternating current controlled over the calling line for deliveiiiig switchcontrol impulses to the automatic switching apparatus over said control circuit, and means responsive to the called subscribers answering the call for rendering said repeater non-responsive to transmit further impulses over said switch-control circuit responsive to further impulses of alternating current.

11. ln a signalling system, a signalling line comprising' two sections, a vacuum-tube repeating device interposed between said sections and including a pair of serially related vacuum tubes, means for transmitting impulses of direct current over the lirst sec.- tion of the line of the iirst vacuum tube, means including the first vacuum tube tor repeating said impulses to the second tube as impulses of alternating current, and means including the second vacuum tube for repeating the impulses of alternating current to the second section of the line as iinpulses of direct current.

` 12. In a telephone system wherein connections are set up through automatic switching apparatus, means for transmitting impulses of alternating` current of a tre quency lying within the voice-frequency range to control the automatic switching apparatus, said automatic switching apparatus being responsive to these impulses to set up the desired connection, and means effective after the automatic switching apparatus has been positioned for rendering the automatic switching apparatus non-responsive to similar impulses of alternating current so as to prevent the established connection being disturbed by voice currents.

13. In a signalling system, a signal repeater, a line incoming to said repeater and the line outgoing therefrom, a vacuum tube in said repeater arranged to operate as a detector tube, a source ot' alternating current associated with said repeater, means controlled over the incoming line tor modulating the alternating current from said source and for transmitting the modulated alternating current to said detector tube, a relay associated with said detector tube, said detector tube being effective to detect the modulations of said alternating current and impress them on said relay, and means controlled by said relay tor transmitting impulses over said outgoing line in accordance with the detected modulations of said alternating current.

14. 1n a signalling system, a repeater having a line incoming thereto and a line outgoing therefrom, a modulator tube in said repeater associated with the incoming line, a detector tube in said repeater associated with the outgoing line, means for transmitting an alternating current from the modulator tube to the detector tube, said modulator tube being arranged to modulate the transmitted alternating current in accordance with signal impulses received over the incoming line, and means controlled by said detector tube for transmitting corresponding signals over the outgoing line.

15. In a telephone system, a subscribers single-conductor, ground-return telephone line terminating in an automatic telephone exchange, a repeater in said exchange interposed between said line and the automatic switching apparatus, means at a substation on said line for transmitting impulses of alternating current corresponding to the digits of a desired telephone number to said repeater, and means in said repeater responsive thereto for transmitting corresponding effective setting impulses to the automatic switching apparatus.

In witness whereof, I hereunto subscribe my name this 26th day ot October, 1931.

CHARLES R. BROOKS. 

